Pulverizer mill



March 7, 1939. H, R. COLLINS 2,149,579

PULVERIZER MILL Filed Oct. 1, 1954 2 Sheets-Sheet 1 INVENTOR -H.R.COLLINS Arm 'EY March 7, 1939. 1 H R co s 7 2,149,579 PULVERIZER MILL IFiled Oct. 1, 1934 2 Sheets-Sheet 2 56 IINVENTOR .R.COLL|NS. BY

{6041* .ATI'ORN Y Patented M... 7, 1939 UNITED STATES PATENT OFFICEPULVERIZER MILL ceased Application October 1,

11 Claims.

This invention relates to pulverizer mills and more particularly tomills of the type in which the grinding devices comprise one or moreannular series of grinding elements, preferably balls, which cooperatewith grinding surfaces, at least one of which rotates in a horizontalplane to actuate the balls. The mill is especially intended for thereduction of non-metallic materials, such as coal, limestones and cementclinker, and is well-adapted for either wet or dry grinding.

It is among the purposes of the invention to produce a. mill of simplemechanical design, and which can be manufactured at much lower cost thanthose now available for similar uses. To this end, the design furthercontemplates the safe use of cast-iron as the principal material ofconstruction, with a minimum number of steel or alloy parts. Among itsmore important mechanical features is the arrangement of the grindingand driving mechanisms as separate as semblies, independently supportedand self-centering. The complete driving mechanism can be assembled andinstalled or removed as a unit without disturbing the grinding elements,which are thus made readily accessible. For purposes of efficiency ingrinding and control'of particle sizes, I prefer to accomplish thereduction of material in at least two stages, by the use of two 30 ormore series of grinding ba'llsand their cooperating grinding surfaces,and contrary to conventional practice to apply the maximum pressure tothe grinding elements of the first stage.

For this purpose, the rotating grinding surfaces or discs are verticallymovable, and their combined weight, together with the weight of theballs of the second or other stages is carried by the grinding balls ofthe first stage. This weight is less than that desirable to effect thefirst stage of reduction, and the required additional pressure isapplied by adjustable spring means, which not only permits theapplication of the most favorable pressures to the secondary stages, butlikewise applies theadditional pressure to the grinding elements of thefirst stage.

Thus the crushing and grinding effect of the first stage is increasedconsiderably as compared with older types of mills, and the finalreduction is accomplished in the secondary stage or stages, whererelatively less pressure is required, with the result that particlesizes are more nearly uniform, and the power requirement relatively low.Further, the pressure applied by the spring means and the weight of theballs and rotating rings is transmitted to the fixed ring, whereby 1934,Serial No. 746,343

the entire grinding assembly may be removed as a unit to expediterepairs or replacement. Thus, instead of a plurality of supports for thegrinding elements, as in conventional practice, the fixed ring is thesole support, and the mill is accordingly a self-centered, ball-bearingmachine. The symmetrical arrangement of both the driving and grindingassemblies about the vertical axis of the mill provides for simplicityin construction, maintained alignment, decreased vibration andconsequently relatively decreased wear of the working parts and quietoperation.

The. driving and speed reduction devices are so designed that the ballsare driven in their race-ways at a speed corresponding approximately tothe rate of fall of material entering the grinding surfaces, for if theballs move too rapidly, they will merely wear out without accomplishingany greater result; and conversely if the speed is too low, coarseparticles falling by gravity will slip between the balls and. pass tothe next stage or out of the mill before the desired reduction is made.a

A vertical arrangement of the grinding and driving assemblies, with thelatter at the top, makes it possible to employ a simple and effectivemeans for wet or dry classification, and forwarding of the pulverizedmaterials. In dry grinding, the classification apparatus has theadvantage that instead of blowing the air stream through the mill, orair sweeping the mill itself, the air is drawn into the mill below thegrinding elements, whereby the fines are discharged without carryingcoarse particles with them. This principal of classification issubstantially preserved in the grinding of damp materials, as the volumeof heated air or gas used for drying is controlled so that the airsweeping of the grinding elements does not occur. A similar arrangementof parts is employed for wet grinding, as will appear more fullyhereinafter.

In general, the preferred form of the new apparatus comprises acylindrical casing, made up of sections which are self-centered, bymeans of Vs and corresponding grooves in adjacent surfaces, thearrangement providing for simplified machining and maintained alignmentof the parts. The lowest section carries the fixed grinding ring, acentral section carries the support for the main bearings of the drivingequipment. The driving mechanism preferably includes a vertical motor ofthe ball-bearing type, secured by its flange to the upper surface of thecasing. The motor shaft may be provided with a pulley for driving afeeder of any suitable type, which will deliver a continuous flow ofmaterial to the grinding elements. The motor shaft also carries ahardened steel pinion, which drives two or made oppositely arrangedidler gears.

The idler gears drive a ring gear, carried by a spider, the latter beingsupported and secured to the main driving shaft of the grindingelements. This shaft is flanged at -its upper end, and countersunk toreceive the upper race of a ball thrust bearing, carried by a supportbetween the central and upper sections of the casing. The upper sectionof the main shaft is cylindrical, and carries a radial bearing arrangedin a central cavity in the said support. Suitable packing is providedbelow the bearing, to preclude the entrance of dust along the main shaftand into the driving assembly. I

The lower section of the main shaft is of square or other angularsection, and is surrounded by two grinding rings or discs, movablevertically thereon, and provided with central openings slightly largerthan the shaft, whereby a limited lateral movement is permitted tomaintain the desired alignment of the grinding surfaces regardless ofinaccuracies in manufacture or the effect of wear, as will appear morefully hereinafter. This limited motion also permits a rotating grindingring to rise to permit the balls to discharge tramp iron or otherforeign material without inJury to the grinding elements.

- The under surface of the upper movable grinding ring is provided witha chilled circumferential grinding surface, curved to correspondsubstantially with the curvature of an annular series of grinding balls.These balls are carried in a similar channel formed in the upper surfaceof a fixed grinding ring, secured between the lower and central sectionsof the casing.

The second stage comprises a similarly curv'ed chilled surface, formedon the under surface of the fixed ring, and it, with a channel formedin. the upper surface of a second movable grinding ring, forms arace-way for the balls of the second stage. i

The two movable grinding rings are drawn together with springrods,-provided with compression springs, whereby the pressure on thegrinding balls may be adjusted in accordance with the characteristics ofthe material to be ground.

For classifying and forwarding dry ground materials, the lower movablegrinding ring is provided with a circular dam, adjustable vertically tocontrol fineness of the material discharged. Air for sweeping andforwarding 'the material enters the mill through a circular series ofports in the casing below the lower rotating ring, due to the action ofa fan, the casing of which closes the lower end of the mill casing; Todrive the fan, a pinion shaft, secured to the motor shaft, is providedwith a coupling hub extension, to

which the fan shaft is secured. This shaft extends through a centralborein the main shaft,

and is centered in a radial bearing in the bottom of the fan casing, thefan being secured to the fan shaft immediately above the bearing. Thefan is thus. driven at the full load'speed of the motor, whichconveniently may be 690 R. P. M., while the balls are driven at a speedcorresponding to the'gravity fall of material, as described above. Bythis arrangement, particle sizes can be closely controlled and theproduct will be much more nearly uniform in size, as compared with millsof other types, as the air stream does not sweep the grinding elements,and carry coarse particles with it.

For wet grinding, the general arrangement of parts is the same. A slurrypump, similarly located and driven, is substituted for the fan; theports in the casing are omitted; and a second ad- Justable dam iscarried on the inner edge of the fixed grinding ring to control thefineness of material discharged from the first stage.

The design also lends itself especially well to the emcient grinding ofdamp materials, such as raw coal, and only minor structural changes andadditions are required. The air inlet ports are surrounded by a volutecasing or wind box, into which air or gas, preferably preheated, fromany suitable source, is forced or drawn into the mill. The air isadmitted in sufficient volume so that,

, in addition to the quantity required by the mill rent is drawnupwardly through the descending material to evaporate the free orsurface moisture. The volume and velocity of the drying air is held tothe minimum necessary for efiicient grinding. The drying air is drawnfrom the mill abovethe grinding elements, by the usual exhaust fan anddischarged into an air separator of any suitable type for recovering anysuspended dust.

It is especially to be realized that by maintaining this low-velocitycounter-current, air sweeping of the grinding elements is avoided, andthat the operation is not to be confused with conventional methods ofair classification or closed-circuit grinding.

For a better understanding of the invention, reference is made to theaccompanying drawings, in which:

Fig. 1 is an elevation, partly in section, of the preferred form of theinvention;

Fig. 2 is a similar elevation, partly fragmentary, of a modified form ofthe mill, adapted for wet grinding, and

Fig. 3 is a similar elevation of the mill as adapted for grinding dampmaterials.

Referring to the drawings, and first to Fig. 1, the apparatus will beseen to consist of a cylindrical casing l, comprising a lower section 2,a central section 3 and an upper section 4, the lowersection 2 beingprovided with an annular fiange 5, which serves as a bed-plate for theapparatus.

A fixed grinding ring 6 is secured between the lower and centralsections, and a main bearing support I is secured between the centraland upper sections, each of the parts above described being centered byV's 8 which correspond with grooves cut into the adjacent surfaces andthe sections together with fixed ring '6 and support I are secured bythe usual through bolts several of which extend only through the lowerfiange of the upper section 4 and the support I. The casing is closed atthe top by means of a coverplate 9, provided with bosses ill, to whichaflange of a vertical motor indicated at H is secured in the usualmanner.

The complete drive assembly is arranged within a compartment provided bythe upper section I, the bearing support I and the cover-plate 9. Themotor shaft is provided with a pulley I 2, to receive a belt l3 fordriving a feeder generally indicated at M, the feeder discharging at.l'into the central section 3. The shaft of the motor is provided with ahardened steel pinion l6, which drives oppositely arranged idler gearsl1 and I8, carried by bearings on suitable vertical shafts, secured tothe cover-plate 9. The idler gears mesh with a ring gear 20, carried bya spider II, the latter being securedto a flange formed on the upper endof the main driving shaft 22. The flange is recessed to receive theupper race-way of a ball thrust bearing 23, the lower race of a which iscarried by the bearing support I.

The upper end of the main driving shaft 22 is cylindrical, and issurrounded by a radial roller bearing 24, arranged in a central bore ofthe sup-- port 1. Below this bearing a groove is provided at 25, toreceive packing of any suitable type, to preclude the entrance of dustalong the shaft and into the driving mechanism. It will thus be seenthat by removing the bolts which pass through the three flanges andlifting the upper section 4, the entire drive assembly can be removed asa unit to permit ready access to the grinding elements, about to bedescribed, and by removing the motor ll an'dthe cover-plate 9, accessmay be had to the various elements of the speed reduction and drivingmechanism.

The lower end of the main shaft 22 is of square or other angular sectionand extends through central openings in the movable grinding rings ordiscs 26 and 21, the openings being slightly larger'than the shaft,whereby the rings are permitted a limited vertical as well as horizontalmovement to maintain alignment regardless ofany inaccuracies inmanufacture or normal wear of the grinding elements. The under surfaceof the grinding ring 26 is provided with a circumferential channel 28,having a curvature corresponding to the annular series of grinding balls29, which travel in a channel formed in the upper surface of the fixedring 6. The under surface of the fixed ring 6 is likewise provided witha circumferential groove or channel corresponding to the surface 28 ofthe movable ring 26, to form the upper race-way for a second series ofballs 30, which are carried in a similar race-way or grinding surface onthe upper surface of the ring 21. Each of the said grinding surfaces orrace-ways is preferably chilled, and

it is to be noted that the design is such that the castings can be madewithout employing angular chills.

The rotating rings 26 and 21 are aper'tured to receive a plurality ofspring rods 3|, carrying compression springs 32, the compression ofwhich may be adjusted by suitable collars and nuts indicated generallyat 33.

It will be seen that the fixed grinding ring 6 carries the weight of therotating grinding rings 26 and 21, together with the grinding balls 29and 30 of the first and second grinding stages, whereby the crushingeffect is vastly greater than in older types of mills. The weight ofthese parts is preferably less than that necessary to effect the initialstage of reduction and the additional pressure is transmitted to theballs 29 of the first stage by the effect of the springs 32, carried bythe spring rods 3|. Accordingly, the pressure on the balls 30 of thesecond stage can be held at the minimum necessary to complete thereduction, and without carrying an excessive load, which would have nosubstantial effect in grinding, but would merely add to the powerrequirement and'the rate of wear. It will be understood that with manymaterials, a second stage of grinding can be carried out moreeconomically by a higher degree of attrition relatively to the crushingefiect, and accordingly although the balls 30 are shown as of the samesize as the balls 29, the second stage may comprise a greater number ofsmaller balls to increase the number of lines of contact.

In order to control the degree of fineness, a.

under side of the mill. base of the mill and the conical section 38 is aare not air-swept, the product will be of unusual uniformity.

Below the rotating ring 21, the lower section 2 .of the mill casing isprovided with a circular series of air ports 36, through which air forsweeping and forwardingthe material is induced by means of the fan 31.The casing of this fan has an upper conical section 38, which closes theInterposed between the grating 39, which may be a perforated plate, thetotal area of the openings "of which is preferably at least equal to thearea of-theports 36, to

decrease the resistance to air flow, the openings being small enough totrap tramp material of sizes liable to damage the fan, such materialbeing removable through the ports 36. The fan is driven by a shaft 40,which passes upwardly through a bore 4| in the main driving shaft 22 andis secured to the hub of the pinion Hi. The lower end of the shaft issupported for rotation in a ball bearing 42, carried in the lowersection 43 of the fan casing It will thus be seen that the motor lldrives the fan- 31 at full load speed, whereas the grinding balls aredriven circumferentially. of the mill at a speed which preferablycorresponds to the gravity fall of material by the effect of the speedreduction gearing above described.

The operation of the apparatus will be evident from the abovedescription, but it is to be realized that the material dischargedfromthe feeder l4 into the central section 3 falls upon the uppersurface of the rotating ring 26, which distributes the material aroundthe grinding elements of the first stage. The falling material passesinwardly through the balls 29, and falls through the space between theinner edge of the fixed ring 6 and the outer surface of the movable ring21, the lower part of which slopes outwardly to guide the material tothe raceway of the balls 30. To vary the pressure of the grindingelements, the compression of the springs 32 is changed by means of thenuts 33, access to which may be through the air, inlet ports 36, and thedegree of fineness finally established by the elevation of the dam 34.

As will appear more clearly from Fig. 2, little change is required inthe parts or their arrangement to adapt the mill for wet grinding. Forthis use, water may be added to the materials entering the mill, orthrough a nozzle indicated generally at 44. An additional circular dam45 is provided and secured adjustably to the inner edge of the fixedgrinding ring 6 to control the fineness of the product discharged fromthe first stage. As compared with the arrangement shown in Fig. 1, thelower section 2 is solid, and a slurry pump 46 is substituted for thefan 31. This pump may likewise be driven in the same manner, and theshaft 40 being similarly carried in a bearing 41 in the bottom of thepump casing.

It will be understood that although I have shown and described thepreferred form of the invention as comprising two grinding stages, thatone or more additional stages can be applied, where the characteristicsof the material to be ground, and the fineness of the product degrindingring 5|.

sired, makes two or more secondary stages desirable. Such an arrangementisshown in Fig. 3, wherein I have also shown the necessary changes andadditions required to adapt the mill for reducing damp materials, suchas raw coal. It is to be understood that these changes and additions arelikewise adaptable to a two-stage mill of the type shown in Fig. 1.

In this arrangement an additional or inter mediate section of the casingis provided at 48 to provide space for the third stage, which comprisesan additional fixed grinding ring 49, having a sloping upper surface 50to divert the descending material to the outer surface of a thirdmovable The under surface of the fixed ring is provided with acircumferential channel, similar to those above described, and theuppersurface of the ring 5| wth a similar channel to form a race-way forthe balls 52. The spring rods 3| are preferably inverted, with thesprings 33 arranged at the top to avoid interference with the rotatingring 5|, and additional spring rods 53, arranged alternately on the samecircle, but of greater length thanthe rods 8|, are provided to supportthe rotating ring 5| and provide the desired pressure on the balls 52.

In view of the increased length of the fan shaft 40, and of the maindriving shaft 22, which has been extended in order to drive the ring 5|,a guide-bearing 54 surrounds the shaft 40 and is retained in a boreformed at the bottom of the shaft 22.

To dry the damp material, preheated air from any suitable source isforced or drawn into a volute casing or wind box 55 and enters the ports36. The lower grinding ring 5i is provided with a circular series ofapertures, 56, and the ring 21 with apertures 51. The area of saidapertures is less than that of the apertures 58, provided in the uppergrinding ring 26, whereby air drawn upwardly through the mill andultimately discharging through the opening 59, due to the action of theusual exhaust fan (not shown), will be divided in its passage throughthe second and third stages, and a portion will pass between the ballsof the said stages. The greater area of the openings 58 causessubstantially all of the air to pass over the baflie 50, formed on theupper surface of the ring 26, the baflie serving both to limit thequantity of fines passing out of the opening 59, and also to preventmaterial entering through the feeder from short-circuiting the firststage through the openings 58. The moisture-- laden air passing out ofthe mill may be conducted from the opening 59 to any suitable type ofair separator (not shown) for recovering any fine particles that may becarried in suspension.

It is to be understood that the actual removal of material from themill, or classification of the final product, is substantially the sameas that described with reference to Fig. 1 and that the quantity ofdrying air is preferably limited to the minimum quantity as influencedby the material moisture content, temperature and humidity of the gas,.as will permit efiicient grinding, in order to prevent actualair-sweeping of the mill.

I claim:

1. A pulverizing mill comprising the combination of a cylindrical casingincluding a lower,

fixed casing section and an upper removable casing section, a grindingassembly arranged in the casing and including a non-rotary grindingring, a rotating grinding ring operatively related thereto, an annularseries of grinding elements operativebetween said rings, and means forexerting yielding grinding pressure on the grinding elements, the lowercasing section being provided with means forming a support for thenon-rotary grinding ring and hence for the grinding assembly, therotating ring having a central opening, a. horizontally extendingsupporting element disposed above the grinding assembly and arrangedintermediate said casing sections and removably supported from the lowercasing section, the supporting element having a central opening andforming with the upper casing section a compartment, 8. drivingmechanism disposed in said compartment and including coaxially arrangeddriving means, driven means supported for rotation on said element, anda main drive shaft secured to said driven means, said drive shaftextending through the openings of said supporting element and therotating grinding ring, and being supported for rotation by said elementand having a driving connection with said rotating grinding ring, thearrangement being such that the driving mechanism'including the maindriving shaft and the supporting element may be raised bodily from thegrinding assembly and the lower cas n section, and that the grindingassembly may thereupon be raised bodily from the lower casing section.

2. A pulverizing mill comprising the combination of a cylindrical casingincluding a lower, fixed casing section and an upper removable casingsection, a grinding assembly arranged in the casing and including anon-rotary grinding ring, a rotating grinding ring operatively relatedthereto, an annular series of grinding elements operative between saidrings, and means for exerting yielding grinding pressure on saidelements, the lower casing section being provided with means forming asupport for the nonrotary grinding ring and hence for the grindingassembly, the rotating ring having a central opening, a driving assemblyfor the rotating ring, means for supporting the driving assembly abovethe grinding assembly including a supporting element arrangedintermediate said casing sections and removable with respect to thelower casing section, thesupporting element having a central opening,the driving assembly including a main driving shaft extending throughthe openings of the element and the rotating ring and having a drivingconnection with the latter, a ring gear having a driving connection withthe shaft and supported for rotation on the supporting element, a sourceof power, and gearing between the power source and the ring gear, thearrangement being such that the driving asssembly including the maindriving shaft and the supporting element may be raised bodily from thegrinding assembly and the lower casing section, and that the grindingassembly may thereupon be raised bodily from the lower casing section.

3. A pulverizer mill comprising the combination of a cylindrical casingcomprising a plurality of vertically arranged removable sections, afixed grinding ring carried by a lower section, a rotating grinding ringoperatively related thereto, an annular series of ginding elementsoperative between the rings, means rotating with the rotating grindingring for exerting yielding grinding pressure on the grinding elementsand for securing the rings and elements together as an assemblyunit, therotating ring having a central opening, a driving assembly for the ringsupported independently thereof, comprising a support carried by asection above the fixed ring, the vertical section above the fixed ring,the support, and the drive means being removably supported by said fixedring, a main drive shaft carried by and extending through the supportand into the openin t engage the walls thereof, and speed reductfimmeans arranged symmetrically about the axis of the mill, including aring gear concentric to the shaft and secured thereto, at least twooppositely arranged idler gears within and meshing with the ring gearand a central pinion for driving the idler gears.

4. A pulverizer mill comprising the combination of a cylindrical casingcomprising a-plurality of vertically arranged sections, a fixed grindingring carried by a lower section, a rotating grinding ring operativelyrelated thereto, an annular series of grinding elements operativebetween the rings, means rotating with the rotating grinding ring forexerting yielding grinding pressure on the grinding elements and forsecuring the rings and elements together as an assembly-unit, the

rotating ring having a central opening, a driving assembly for the ringsupported. independently thereof, comprising a support carried-by asection above the fixed ring, a main drive shaft extending through thesupport and into the opening to engage the walls thereof, and speedreduction means arranged symmetrically about the axis of the mill,including a spider connected to the shaft and carrying a ring gear, atleast two oppositely arranged idler. gears meshing with the ring gear,each of said gears being enclosed within another section of the casingabove the support, the section having a cover-plate enclosing the mill,a vertical motor carried by said cover-plate and secured centrallythereof, the motor having a shaft extending through the cover-plate andcarrying a pinion meshing with the idler gears for driving the latter.

5. A pulverizer millcomprising the combination of a casing, a fixedgrinding ring carried by the casing, a rotating grinding ringoperatively related thereto, an annular series of grinding elementsoperative between the rings, the rotating grinding ring having a centralopening, a fan for forwarding pulverized material and means for drivingthe rotating grinding ring at relatively slow speed and the fan atrelatively high speed from the same source of power comprising a maindriving shaft extending into the opening in the rotating grinding ringand engaging the walls thereof, speed reduction means for driving theshaft, the shaft having a central bore, a fan shaft extending through"the bore and connected to the fan, and common means for driving the fanshaft and the speed reduction means.

6. A pulverizer mill comprising the combination of a casing, a fixedgrinding ring carried by the casing a rotating grinding ringoperatively.

related thereto, an annular series of grinding elements operativebetween the rings, the rotating grinding ring having a central opening,a fan below the grinding rings for forwarding pulverized material, andmeans above the rings, for driving the rotating ring at relatively slowspeed and the fan at relatively high speed from the same source ofpower, comprising a main driving shaftextending downwardly into theopening in the rotating grinding ring and engaging the walls thereof,speed reduction means for. driving the shaft, the shaft having a centralbore, a fan shaft extending through the bore and connected to the fan,and common means for driving the fan shaftand the speed reduction means.

'7. A pulverizer mill comprising the combination of a casing, a fixedgrinding ring carried by I the casing a rotating grinding ring'operatively related thereto,; an annular series of grinding elementsoperative between the rings, the rotat-, ing grinding ring having acentral opening, discharge means, for pulverized material, below'thefixed'ring, and means above the rotating ring for driving therotatinggrinding ring at relatively slow speed and the discharge means atrelatively high speed from the same source of power. comprising a maindriving shaft, extending downwardly into the opening in the rotatinggrinding ring and engaging the walls thereof, speed reduction means fordriving the shaft, theshaft having a central bore, a second shaftextending through the bore and connected to the discharge means, andcommon means for driving said second shaft and the speed reductionmeans.

8.- A pulverizer mill comprising the combination of a casing, a fixedgrinding ring carried by the casing and having a grinding surface on itslower side, an annular series of grinding elements engaging saidsurface, a rotating grinding ring.

supporting said grinding elements, means'for delivering materialthereto, means to drive the rotating grinding ring, a cylindricalclassifying dam secured to the peripheral portion of the rotatinggrinding ring and on the discharge side thereof, the-dam beingvertically adjustable to control the degree of fineness of the groundmaterial discharging thereover, the'casing being provided with airinlets located immediately below the rotating grinding ring, a fanlocated below said inlets and having its suction side opening into thelower portion of the casing whereby to induce an air stream through saidinlets and into which the ground material discharging over said damfalls, said fan being con-- centric with said rotating ring drivingmeans and being driven thereby.

.9. A gravity-discharge pulverizermill form ducing damp materials,having at least two grind-' ing stages comprising the combination of acasing, a fixed grinding ring carried by the casing and havinga centralpassage for air and material and a grinding surface on its upper side,an annularseries of grinding elements on said sufface, a rotatinggrinding ring resting upon and carried by said elements, a secondrotating grinding ring below and yi'eldingly supported bythe first andhaving a grinding surface on its upper side, a second series of grindingelements on said surface in engagement with an under surface of thefixed ring, each of said rotating rings being provided with openingswithin said central passage for 'the flow of air, the casing beingprovided with an air inlet below the second grinding ring adapted toadmit drying air, the casing being also provided with an outlet abovethe rotating rings through which at least apart of the air entering theinlet may be drawn to provide a counter-current of air fordriving thematerial, the material passing downwardly through said grinding stagesin opposition to the countercurrent.

10. A pulverizer mill comprising the combination of a casing, a fixedgrinding ring carriedby the casing and having a grinding surface on itslower side, an annular'series of grinding elements engaging saidsurface, a rotating grinding ring having a rim portionprovided with asubstantially horizontal upper face, a'grinding surface.

grinding elements, means rotating with the rotating ring for exerting ayielding upward pressure on the rotating grinding ring and grindingelements supported thereby, means for feeding material to be pulverizedto the inner side of the grinding elements for movement outwardly acrossthe grinding surfaces, a cylindrical element removably mounted on theouter peripheral face of the rotating grinding ring and extending abovethe level of said peripheral edge, said element functioning as a dam forretarding discharge of the pulverized material and being verticallyadjustable with reference to the rotating grinding ring.

11. A pulverizer mill comprising the combination of a cylindrical casingincluding a lower fixed casing section, an upper removable casingsection, and a member arranged intermediate said sections and removablysupported from the lower section, the intermediate member dividing thecasing into upper and lower compartments and having a drive shaftopening, a grinding assembly including a non-rotary grinding ring, arotating grinding ring having a drive shaft opening therethrough, saidrings each having a circumferential ball channel and said channels beingvertically aligned, an annular series of grinding balls disposed in saidchannels, and means rotating with the rotating grinding ring for exerting yielding grinding pressure on the grinding balls and for securingthe rings and balls together as an assembly-unit, means including thelower casing section providing a support for the non-rotary ring andhence for the grinding assembly-unit, from which support the grindingassembly-unit is readily removable, a driving assembly contained in theupper compartment and supported by the intermediate member, the drivingassembly including a drive shaft which extends downwardly through theaforesaid shaft openings and has a driving connection with the rotatinggrinding ring, means for directing material to be pulverized to thegrinding assembly including a casing inlet disposed below theintermediate member, the lower casing section having an opening for thedischarge of pulverized material from the casing, and the arrangementbeing such that the upper casing section may be removed to provideaccess to the driving assembly and that the upper section, the drivingassembly, and the intermediate member may be raised bodily from thecasing, and the grinding assemblyuniii thereupon raised from the lowercasing section.

HARRY R. COLLINS.

